Then set one new truss in place against the old roof and use it to snap chalk lines to mark the location on the old roof for diagonal 2 inch by 6 inch valley boards which will hold jack rafters to tie the roofs together.
Old roof new roof joined rafters.
Installing a new roof over an existing one also called a layover or nailover is often possible although not always the best option.
Staple on roofing paper and shingle the new shed roof starting at the bottom and working up.
Tie a new gable or hip roof to an existing gable or hip roof with valley rafters which perform the same function as ledgers.
Strip the roofing from the old roof at the connection point to.
You can strip the shingles out of the area that the new roof will cover but to be safe i often leave the old shingles in place and frame over them.
Cover the roof with roofing paper overlapping seams generously then nail on shingles.
The new roof will not touch the old one in any way only sit on the wall plates with perhaps two feet of air space between old and new roofs.
Put siding on the new roof end replacing any removed from the old roof area.
Reinstall the removed shingles or use new shingles to cover the area of the old roof which was exposed.
This new roof with a higher pitch can be accomplished with premade roof trusses and typical asphalt shingles.
The new plywood will have to be nailed to the rafters or roof trusses.
Install bent metal flashing where the old and new roofs meet and on the peaks and metal drip edges along the eaves.
Add decking atop the roof and nail on metal flashing where the shed roof joins the existing roof.
First set the new rafters or trusses on the addition walls up to the point of the eave of the main house and snap chalklines on the shingles to mark the center of the new valley.
How to install a roof over an existing roof.
Nail oriented strand board osb decking over all the new rafters.
But with the old plywood left in place it will be considerably more difficult to locate the rafters to ensure proper nail.